There are not a lot of hard, fast rules when it comes to these early Nintendo cabs. There are all sorts of variations out there. When the parts are essentially swappable it makes sense that the factory may have used what was on hand when it was needed over using inflexible rules on what parts go with what machine.
Also, as time went on it seems like Nintendo standardized a lot of its parts for operational simplicity. One very plausible theory is that when the cab shape changed to make Radar Scope it was operationally simpler to put Hellifire in the newer cab style. Now you can produce both game in the same cab and more easily make cabs to order.